1. Field of the Invention
This invention is a rotatable device for holding at least two substrates, such as turbine engine parts, whereby each substrate may also be rotated about its individual axis during a high temperature vacuum coating process.
2. Brief Description of the Prior Art
In coating turbine parts, it is desirable to control the thickness distribution of the deposited coating. The desired distribution may be uniform or variable over the surface of the parts. One way in which the distribution may be controlled is by proper positioning of the part with respect to the coating source. In particular, the part can be held at an angle to the path of the coating material so that more material is deposited at one end than at the other.
Previously, it was known to mount a number of turbine parts on a holder at the end of a main shaft which extended through a load lock into a vacuum chamber. Each part was mounted on a stub at an angle, e.g., 20.degree., with respect to the main shaft. The main shaft was rotated about its longitudinal axis so that each part was exposed equally to the coating source. However, in order to obtain the desired thickness distribution, it was necessary to periodically remove the holder and the parts from the vacuum chamber and manually turn the parts 180.degree. about their mounting stub on the holder.
Periodic removal of the parts during the coating process has several distinct disadvantages. First, removal extends the processing time because of the time required to cool and reheat the parts, and vent and re-evacuate the load lock. Second, additional manual labor was required to demount, rotate, and remount the parts on the holder. Third, and most important, cooling and exposure of a partially coated part to the atmosphere causes an interface in the coating layer which may have significant detrimental effects in the life time and other performance characteristics of the coating. Despite the practical disadvantages of periodically removing the parts during the coating process, this procedure was followed for several years because it was believed impractical to rotate the parts with respect to the holder while in the coating position. It must be remembered that the turbine engine parts are typically coated at temperatures exceeding 800.degree. C in vacuum lower than 10.sup.-4 Torr. Under such conditions contacting metal surfaces tend to seize and lubricants which might contaminate the coating must be avoided. Further, the coating material tends to quickly clog and foul exposed moving parts.